If the health insurance didn't cover birth control, could failure to purchase birth control on their own void their coverage if they get pregnant? If so, then failure to cover contraceptives would definitely reduce the cost of health insurance.
Facetious, perhaps, but it's really hard to compare health insurance to other types of insurance when health insurance typically covers planned events, such as pregnancy. If it were purely insurance against unplanned sickness, accidents, etc, then I guess many more people would be delaying childbirth one way or another - or doctors/hospitals would raise the rates for covered events even higher to cover the losses they were absorbing when people who can't afford kids have kids anyway.